1. Field of the Invention
The present invention relates to a novel process for the complete and selective removal of low molecular weight dimeric species such as p-cresol dimer, from phenolic polymers, particularly from novolak condensates, and to photoresist compositions containing such dimer-free novalak polymers.
2. Description of Related Art
Novolak condensates or resins are formed by reacting phenolic compounds with aldehydes such as formaldehyde in the presence of an acid catalyst. Novolaks formed from mixtures of p-cresol and p-cresol are particularly useful as binders in photoresist compositions because of their excellent resistance to plasma-etching and their high alkali solubility.
It is known to those skilled in the art of photoresist compositions that the use of larger proportions of p-cresol, as the phenolic reactant in novolaks formed from mixtures of o-cresol, p-cresol and p-cresol, can improve the lithographic performance of the photoresist images, particularly with respect to resolution and image profile. However, it has been found that the incorporation of larger amounts of p-cresol results in an increase in the amount of insoluble residue, or scum, remaining in the image areas, which residue or scum cannot be removed by the development process. Analytical studies have shown that this residue or scum is largely caused by the presence of phenolic dimers, particularly the dimer of p-cresol, in novolaks formed from phenolic compounds comprising p-cresol, as shown by gel-permeation chromotography.